Method and device for emitting radial seismic waves in a material medium by electromagnetic induction

ABSTRACT

Method and device for emitting radial seismic waves in a material medium by electromagnetic induction, used notably for generating seismic waves in cased or uncased wells or in a water mass.  
     Emission of radial waves is essentially obtained by radially expanding a metal tube ( 1 ) under the effect of a magnetic pressure generated by electromagnetic induction, elastic waves being created in the medium under the effect of this expansion. The magnetic pressure is obtained by connecting a coil ( 2 ) in line with the tube to a current generator ( 3 ): shock generator, variable-frequency generator. The winding pitch of coil ( 2 ) can be constant or variable. Tube ( 1 ) can for example be added into a well or hole or it can be a tube portion of a cased well. It can also be the lateral wall of a sealed enclosure that is immersed in a water mass so as to produce acoustic or seismic waves therein.  
     Applications: seismic prospecting or monitoring of the subsoil for example.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and to a device foremitting radial seismic waves in a material medium such as the subsoil,by electromagnetic induction, used notably for generating seismic wavesin cased wells or in a water mass.

[0002] The device according to the invention finds applications notablyfor operations of seismic prospecting or monitoring of the subsoilwherein seismic wave emission is conventionally triggered and the wavesreflected by the formation discontinuities are recorded by means ofseismic receivers (geophones, hydrophones) in order to form seismograms.The device can be used for example to generate seismic waves in cased oruncased wells or in a water mass.

BACKGROUND OF THE INVENTION

[0003] There are different types of seismic sources suited to emit inwells. They involve:

[0004] either an explosion: point or elongated charge, detonating cordhelically wound on a rigid spindle,

[0005] or an electric spark (breakdown between electrodes in water, wireexplosion under the effect of an electric discharge, etc.),

[0006] or a vertical mechanical shock of a mass falling or thrown ontoan anvil secured to a packer, which produces a vertical shear on thewell wall mainly generating S waves,

[0007] or a horizontal shock, in a radial direction, of a mass radiallydriven by hydraulic or electromagnetic means and that strikes the wallof the well at one point.

[0008] Controlled vibrational sources of piezoelectric ormagnetostrictive type, coupled (or not) with the well wall, which emitmono-frequencies or signals that are coded or frequency-modulated by aramp, are also used to create seismic waves in wells.

[0009] These sources can be used in a cased or an uncased well. In thecase of cased wells, their efficiency is affected by the stiffness ofthe casing which limits the stress applied to the surrounding medium.

SUMMARY OF THE INVENTION

[0010] The device according to the invention uses a well-known physicalprinciple, i.e. the motive action provided by an impulsive magneticfield already used in other spheres, for example:

[0011] the making of marine acoustic sources where the repulsion of twometal disks surrounding a flat coil fed by an electric shock generator(plane structure) is used,

[0012] the making of electromagnetic shutters for electromagneticradiations (optical or X-ray spectrum) consisting of a thin metal tubeplaced in line with and inside a coil whose impulsive magnetic fieldcauses collapse,

[0013] plasma acceleration, etc.

[0014] The method according to the invention allows to generate radialelastic waves in a material medium. It essentially consists in radiallyexpanding at least part of the wall of a metal tube in contact with themedium under the effect of a magnetic pressure generated byelectromagnetic induction, with emission in the medium of the elasticwaves created in the medium under the effect of this expansion.

[0015] The device according to the invention allows to generate radialelastic waves in a material medium. It essentially comprises a metaltube in contact with the medium and motive means arranged inside thetube to exert either an isotropic magnetic pressure on all of the wallof the tube, thus causing a radial expansion of the wall of the metaltube, or an anisotropic magnetic pressure on the wall of the tube,causing an (anisotropic) radial expansion of only part of the wall ofthe metal tube.

[0016] According to a first embodiment, the motive means comprise a coilformed for example on an insulating spindle with a constant or avariable winding pitch that can increase from the central part to theends of the coil so as to increase the dipolar radiation along the axisof the coil, or decrease from the central part to the ends of the coilso as to modify the acoustic radiation diagram as a function of thefrequency.

[0017] According to another embodiment, the motive means comprise atleast one coil formed on a cylindrical wall portion of a spindle,intended to create an anisotropic magnetic pressure exerted on at leasta cylindrical portion of the tube.

[0018] The coil can comprise a core of high magnetic permeability and oflow coercive force.

[0019] The electric generator can be a shock generator suited to providecurrent pulses or a generator suited to provide current pulse trains soas to generate vibrations in the medium. This pulse train generator canfor example be controlled by a control element suited to generate avariable-frequency control signal.

[0020] The tube is for example a well casing tube mechanically coupledwith the formations surrounding the well and the motive means cancomprise a sonde connected to an electric excitation generator, thissonde being suited to be moved in the well up to the triggering point.

[0021] According to an embodiment, the electric generator comprises forexample a capacitor battery arranged in an enclosure in the vicinity ofthe sonde, supplied by an electric source at a distance from theenclosure.

[0022] According to an embodiment, the tube is the lateral wall of asealed enclosure, the motive means comprising a coil placed in theenclosure, the electric generator being arranged at least partly outsidethe enclosure.

[0023] The device can be used for example within the scope of operationsof onshore seismic exploration or monitoring of an undergroundreservoir. Waves are emitted in the formations surrounding the well bytriggering electric generator 3, waves reflected by the underground zonediscontinuities are received and recorded, and the records are processedso as to form seismograms of the zone.

[0024] The device can also be used for example within the scope ofoperations of marine seismic exploration or monitoring of an undergroundzone below a water mass, with immersion of enclosure 19 (see FIG. 10)from a vehicle or a stationary installation 20, emission of waves in thewater mass by triggering electric generator 3, reception and recordingof the waves reflected by the underground zone discontinuities andprocessing of the records so as to form seismograms of the zone.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Other features and advantages of the method and of the deviceaccording to the invention will be clear from reading the descriptionhereafter of non limitative impulsive source examples, with reference tothe accompanying drawings wherein:

[0026]FIG. 1 diagrammatically shows the principle of the device with anelectromagnetic current pulse generator for creating an expansion in atube,

[0027]FIG. 2 shows the simplified equivalent circuit of the currentgenerator,

[0028]FIG. 3 diagrammatically shows the distribution of the mechanicalstresses,

[0029]FIG. 4 shows a practical embodiment example of the pulsegenerator,

[0030]FIG. 5 shows an example of variation as a function of time of thevoltage applied to the primary coil of the pulse generator,

[0031]FIG. 6 shows an example of variation as a function of time of thecorresponding electric current circulating in the tube,

[0032]FIG. 7 shows an example of a seismic signal received by a wavepickup such as a geophone at a given distance from the seismic source,

[0033]FIG. 8 shows the frequency spectrum of the seismic signal of FIG.7,

[0034]FIG. 9 diagrammatically shows a variant of the embodiment of FIG.4,

[0035]FIG. 10 diagrammatically shows an application of the device forgenerating acoustic waves in water,

[0036]FIG. 11 shows an embodiment using flat coils allowing to create ananisotropic dipolar source, and

[0037]FIG. 12 illustrates the effect on the tube of the current appliedto the coils of FIG. 11.

DETAILED DESCRIPTION

[0038] In order to generate seismic waves in a material medium, avariation of the cross-section of a metal tube 1 in contact with themedium is caused as mentioned above, i.e., in the example describedhere, a radial expansion of tube 1 under the effect of a magneticpressure generated by electromagnetic induction.

[0039] This magnetic pressure is created (FIG. 1) by means of a coaxialcoil 2 with N spires placed inside tube 1, to which an intense electriccurrent pulse I₁ produced by an electric shock generator 3 is applied,which generates an axial magnetic field H. An induced current I₂ whichtravels metal tube 1 in a circle is created by induction. Axial magneticfield H produces on any element of tube 1 through which current I₂ runsan orthogonal electromagnetic force F headed radially. The tube behaveslocally like a single spire closed on itself.

[0040] The coil/casing system behaves like an air transformer (FIG. 2)with a primary winding LP (coil 3) and a secondary winding LS (tube 1).In order that the induced current$\left( {e = \frac{\Phi}{t}} \right)$

[0041] is maximum, a fast flux variation is required. The electricdischarge provided can be provided by the discharge of capacitors C.

[0042] Electric shock generator 3 comprises for example (FIG. 2) acontinuous supply 4, a capacitor battery 5, a spark gap 6 ensuringcurrent change-over in coil 2.

[0043] Over a large part of the coil length, the waves generated arecompression waves PW (FIG. 3). In the vicinity of the opposite terminalparts of coil 2, the magnetic field created is dipolar and the wavesgenerated are also S type waves.

[0044] In order that this process develops with an acceptableefficiency, the excitation coil/casing magnetic coupling has to be closeto 1, which implies that the coil is long in relation to its diameter Dand that this diameter is close to the inside diameter of the tube orcasing. A coil of length L such that D/L<0.2 is for example selected.

[0045] The flux variation also has to be very fast for the induction inthe secondary circuit to be as intense as possible and the surface ofthe tube or casing subjected to the radial stress must be sufficient toensure an elastic radiation in the desired frequency range.

[0046] The shock generator is suited to the equivalent circuit of thesource. The electric circuit of shock generator 3 mainly consists of aself-inducting coil Lp and of a resistor. In order to keep the impulsivemode and to avoid an oscillating discharge whose polarity reversals areharmful to the capacitors of the electric shock generator, the systemhas to be set to critical damping.

[0047] The equivalent electric circuit of the seismic source is atransformer whose secondary (the tube) is closed on itself. Theimpedance at the primary of the transformer is that of the secondarymultiplied by the square of the transformation ratio. The transformationratio being N, the impedance at the primary is Z_(p)=N².Z_(s).Determination of N depends on the characteristics of the tube used andon the equivalent capacity of the discharge circuit.

[0048] The dominant frequency depends on the electric resonance of theequivalent circuit. For a given energy and central frequency, thecapacity of the electric shock generator and its charging voltagedetermine the value of the potential energy of the system. Theself-inducting coil of the source itself must be dimensioned (length,number of spires, etc.) to obtain the desired mean frequency.

[0049] Tube 1 can be, for example, a casing and coaxial coil 2 ispositioned inside the tube at the depth where it is desired to generateradial magnetic forces.

[0050] The well seismic source illustrated in FIG. 4 comprises a rigidspindle 7 made of an insulating material on which a solenoid 8 is woundand an electric cable 9 connecting coil 8 to shock generator 3. Electriccable 9 is for example a coaxial cable, the opposite ends of coil 8being respectively connected to central conductor 10 and to shield 11.The electric cable connecting coil 8 to shock generator 3 must be asshort as possible to prevent losses. If the seismic source is intendedfor seismic prospecting operations in relatively deep wells W (typicallyof the order of 200 m or more), shock generator 3 is divided into twoparts. The seismic source is suspended by a cable portion 12 from acontainer 13 where capacitor battery 5 and spark gap 6 are placed.Another electric cable 14 connects container 13 to an assembly 15 placedfor example at the surface and comprising electric source 4 and atrigger circuit 16 (see FIG. 2).

[0051] Triggering of the electric shock is precise (uncertainty of theorder of 1 micro-second) and the quasi-absence of mechanical motion,except for the expansion of the tube, allows excellent synchronizationand good repetitiveness of the signature of the signal emitted.

EMBODIMENT EXAMPLE

[0052] A coil was made for a 7-inch casing and a shock generator withcapacitive storage of 1 kJ was associated therewith (C=80 μF, V=5 kV).The coil, which was 145 mm in mean diameter (150 mm in overall diameter)comprised 200 spires in a single layer over a length of 1 m. Theresistance of the loop consisting of 1 m of the casing was of the orderof 10⁻⁴Ω.

[0053]FIGS. 5 and 6 respectively show the shape of the signal at theoutput of the electric shock generator and that of the currentcirculating in the tube or casing. The pulse has a waveform close to theone desired at critical damping. The current peak is of the order of 300kA, which gives a pressure peak of the order of 60 kPa.

[0054] The emitted seismic signal measured by a geophone in a well 5 mabove the source is shown in FIG. 7. Its spectrum is in accordance withthat of the electric pulse with a maximum amplitude in the vicinity of600 Hz (FIG. 8).

[0055] Variants

[0056] In order to improve the efficiency of the source, the axialmagnetic field can be increased by placing a ferromagnetic core 17 (seeFIG. 4) in the excitation coil so as to improve the mutual coupling ofthe solenoid with the tube or the casing element.

[0057] Because of the operating mode, it is essential that this core 17exhibits low losses (hysteresis, convection currents) and that itsmechanical rigidity is high to ensure geometrical stability of the coil.A core 17 made of ceramic for example (ferromagnetic ferrite with a lowcoercive force) meets such requirements. It lends itself to the windingof the solenoid embedded in the surface of the spindle (made of ceramicfor example) for a better dimensional stability of the coil (higherresistance to the magnetic forces of which it is itself the seat).

[0058] Other variants

[0059] It is also possible to make a variable-pitch winding (2 or 8) toweight the magnetic pressure along the tube in order to regulate theacoustic radiation diagram. The pitch can for example decreasesymmetrically from the middle of coil 2 to modify the acoustic radiationdiagram as a function of the frequency, or increase symmetrically with anarrower pitch in the vicinity of the ends so as to increase the dipolarradiation along the axis of the coil.

[0060] According to the embodiment of FIGS. 11, 12, the motive meanscomprise two curved flats coils or pancake coils 22A, 22B formed on twoopposite portions of the lateral wall of a spindle 23 (by embedding ongrooved wall portions for example). With this layout, application ofelectric currents to these coils creates radial forces on two oppositewall portions 24 of tube 1. A dipolar anisotropic source is thuscreated.

[0061] For applications in a cased well, tube 1 is the casing of thewell itself. In other cases where the device is placed in an uncasedwell or in a cavity formed in the medium, it comprises an external tubeelement or a cylindrical metal shell that contains the electricexcitation circuits.

[0062] Examples where the material medium in contact with tube 1 is asolid medium have been described so far. The device could also be usedfor emitting radial elastic waves in water (FIG. 10) without departingfrom the scope of the invention. The spindle with its outer coil 8, 21as described in FIGS. 4 or 12 is placed in a tube 19 closed at both endsand electrically insulated from the outside medium. The device can befastened to the hull of a ship or to a floating structure 20, or towedin immersion by a towline 21 which can be, for example, the coaxialfeeding cable such as cable 9.

[0063] A control mode intended to produce seismic impulsive signals hasalso been described. It is however clear that, by feeding coil (2, 8) bymeans of longer periodic electric signals, vibrations can also begenerated in the medium surrounding the source. A control element 18(see FIG. 9) suited to generate variable-frequency control signals andthus to obtain increasing or decreasing frequency vibrations, with alinear or logarithmic ramp, or a succession of mono-frequencies, isused. In such a case, the intensity of the electric signals can also beexploited to raise the low frequency level if need be.

[0064] The device can be used for example for seismic prospectingoperations as well as for seismic monitoring operations in a hydrocarbonreservoir during production or development of an underground fluidstorage reservoir. It is particularly well-suited notably for seismiccrosshole tomography.

[0065] The seismic trace processing operations conventionally comprisecorrelating the seismic signals reflected by the discontinuities of themedium explored by the control pilot signal of the vibrator. The radialstress exerted on the surrounding medium being always exerted in thesame direction here, whatever the polarity of the magnetic field, eitherthe pilot signal after rectification or the square of the pilot signalis used as the reference signal.

1. A method of generating radial seismic waves at any point of at least a portion of a well through a geologic formation through the effect of a magnetic pressure generated by electromagnetic induction, characterized in that a metal tube (1) tightly coupled with the formations surrounding the well is installed all along said well portion and an electromagnetic generator is lowered into the well to a set point in order to locally expand the tube through the effect of a magnetic pressure, with emission in the medium of the seismic waves created in the medium under the effect of this expansion.
 2. A seismic emission device for generating radial seismic waves at any point of a well through a geologic formation through the effect of a magnetic pressure generated by electromagnetic induction, characterized in that, the well being cased over part of the length thereof by a metal tube (1) tightly coupled with the surrounding formations, the seismic enission device comprises a well tool connected by an electric carrying cable (14) to means for moving it all along the cased part of the well, the tool comprising an elongated spindle (7) on which are wound coils (8) and an electric generator (13, 15) in order to generate current pulses in the coils and to cause, by magnetic pressure, local expansion of the casing tube with emission of seismic waves in the formation.
 3. A seismic emission device for generating radial seismic waves at any point of a zone of a well through a geologic formation through the effect of a magnetic pressure generated by electromagnetic induction, characterized in that it comprises a metal tube (1) tightly coupled with the formations surrounding the total length of the zone and a well tool connected by an electric carrying cable (14) to means for moving it along the well zone, the tool comprising an elongated spindle (7) on which are wound coils (8, 22) and an electric generator (13, 15) intended to generate current pulses in the coils and to cause, by magnetic pressure, local expansion of the casing tube with emission of seismic waves in the formation.
 4. A seismic emission device as claimed in claim 2 or 3 , characterized in that coils (8) are formed on insulating spindle (7) with a constant winding pitch.
 5. A seismic emission device as claimed in claim 2 or 3 , characterized in that coils (8) are formed on insulating spindle (7) with an increasing winding pitch from the central part to the ends of the coil, so as to increase the dipolar radiation along the axis of the coil.
 6. A seismic emission device as claimed in claim 2 or 3 , characterized in that coils (8) are formed on insulating spindle (7) with a decreasing winding pitch from the central part to the ends of the coil, so as to modify the acoustic radiation diagram as a function of the frequency.
 7. A seismic emission device as claimed in claim 2 or 3 , characterized in that the coils comprise at least one coil (22) formed on a cylindrical wall portion of a spindle (23), intended to create a magnetic pressure exerted on at least a cylindrical portion (24) of tube (1).
 8. A seismic emission device as claimed in any one of claims 2 to 7 , characterized in that coil (8, 21) comprises a core (17) of high magnetic permeability and low coercive strength.
 9. A seismic emission device as claimed in any one of claims 2 to 8 , characterized in that the electric generator comprises a capacitor battery (5) arranged in an enclosure (13) in the vicinity of sonde (WS), fed by an electric seismic emission device (5) at a distance from enclosure (13).
 10. A seismic emission device as claimed in any one of claims 2 to 8 , characterized in that electric generator (3) is a generator suited to supply current pulse trains so as to generate vibrations in the medium.
 11. A seismic emission device as claimed in claim 10 , characterized in that electric generator (3) comprises a control element (18) suited to generate a variable-frequency control signal.
 12. Application of the seismic emission device as claimed in any one of claims 2 to 11 within the scope of onshore seismic exploration or monitoring operations in an underground zone, with emission of waves in the formations surrounding the well by triggering electric generator (3), reception and recording of the waves reflected by the discontinuities of the underground zone and processing the records so as to form seismograms of said zone. 